c - X11 Wait for and Get Clipboard Text

Question

I have to monitor the X11 Clipboard.

For the moment, I request the ClipBoard Selection each 5 seconds, then I hash the text returned from clipboard and I compare it with the hash calculate from the last check. If hash are not the same, I analysis the text content and do some stuff...

I don't like my method. I'm from Windows, and with the winapi, it is the kernel that notify your program when the clipboard has changed, and it's more efficient!

I just want to know if it is possible that X11 can notify your program as winapi when the clipboard has changed ? What is the more efficient way to check clipboard modifications with X11 ?

Answer 1

Use XFixesSelectSelectionInput() from Xfixes extension and wait for XFixesSelectionNotify event.

Example:

// gcc -o xclipwatch xclipwatch.c -lX11 -lXfixes
...
#include <X11/extensions/Xfixes.h>
...
void WatchSelection(Display *display, Window window, const char *bufname)
{
  int event_base, error_base;
  XEvent event;
  Atom bufid = XInternAtom(display, bufname, False);

  assert( XFixesQueryExtension(display, &event_base, &error_base) );
  XFixesSelectSelectionInput(display, DefaultRootWindow(display), bufid, XFixesSetSelectionOwnerNotifyMask);

  while (True)
  {
    XNextEvent(display, &event);

    if (event.type == event_base + XFixesSelectionNotify &&
        ((XFixesSelectionNotifyEvent*)&event)->selection == bufid)
    {
      if (!PrintSelection(display, window, bufname, "UTF8_STRING"))
        PrintSelection(display, window, bufname, "STRING");

      fflush(stdout);
    }
  }
}
...

This works both for bufname == "CLIPBOARD" and bufname == "PRIMARY" selection.

Also see PrintSelection() function in this answer.